1. Field of the Invention
The present invention relates to a noise reducer and noise reducing method for removing noise from a video signal, to a video signal display apparatus using the noise reducer or noise reducing method, and more particularly to a noise reducer, noise reducing method, and video signal display apparatus in which motion sensitivity and noise sensitivity can be set independently.
2. Description of the Related Art
Because of the increasing size of the screens of television receivers and other display apparatus and the increasing quality of the pictures displayed on them, high reliability is required in video signal processing; if an input video signal or image signal includes unnecessary signal components, generally referred to as noise components, the resulting image blemishes can be glaringly conspicuous. Noise reduction (NR) apparatus exists for improving the quality of video signals by reducing their noise components: for example, since noise normally lacks frame-to-frame coherence (correlation from frame to frame on the time axis), many noise reducers that cancel signal components lacking such coherence have been proposed. These noise reducers are described as three dimensional (3D) because they operate in the time dimension as well as in two spatial dimensions.
A 3D noise reducer basically operates by taking the difference between the video signals in the current frame and the preceding frame, multiplying the resulting frame-to-frame difference signal by a so-called recursion coefficient, and subtracting the product from the video signal of the current frame (or adding the product, if the recursion coefficient is negative). A problem with this basic scheme is that although it reduces noise, it also generates motion artifacts: moving parts of the image may be smeared, sometimes acquiring comet-like tails, and after-images may appear. To reduce these motion artifacts, many known noise reducers detect motion in the video signal from, for example, the frame-to-frame difference value and reduce the recursion coefficient in image areas where motion is present. The following Japanese Patent Application Publications (JP) disclose noise reducers of this type.
JP 2003-219208 (FIGS. 1 and 4) discloses a noise reducer that distinguishes between noise and motion by detecting the maximum and mean values of the frame-to-frame difference data over a certain interval, and generates setting information and a control signal for on-off control of noise reduction from the detected maximum and mean values.
JP 2002-33942 (FIGS. 1 and 3) discloses a noise reducer that detects the differences between picture element (pixel) signals over a plurality of frames as a motion index and sets the noise recursion gain accordingly.
JP 2005-347821 (FIGS. 1 and 2) discloses a noise reducer that performs noise reducing processing according to image-brightness by controlling the noise recursion coefficient according to both the result of motion detection from the frame-to-frame difference and the luminance level of the image signal.
JP 9-81754 (FIG. 1) discloses a noise reducer that makes motion decisions on the basis of frame-to-frame differences, checks the results of the decisions with a majority rule circuit, corrects scatter in the decision results, and uses the corrected results as a motion signal to determine the noise recursion coefficient, a scheme that is good at detecting motion.
JP 2004-96628 (FIG. 3) discloses a noise reducer that uses motion vectors to obtain a motion-corrected field-to-field difference signal, detects motion separately in the high and low frequency bands of this signal, and controls the amount of feedback in the high-frequency band to obtain improved after-image reduction.
Although these conventional noise reducers mitigate motion artifacts to some extent, they still suffer from a common problem. The frame-to-frame difference includes both differences due to motion and differences due to noise. If the motion detection sensitivity is increased, noise may be mistakenly detected as motion, with the result that noise reduction is suppressed precisely when it is needed. If the motion detection sensitivity is reduced, however, not only do visible motion artifacts remain, but when a video signal with large noise values due to automatic gain control is processed, noise is still mistakenly detected as motion, so that prominent noise components are not reduced.